1. Field of the Invention
The present invention relates to a II-VI group compound semiconductor device and a method for manufacturing the same. More particularly it relates to a II-VI group compound semiconductor device having an electrode structure with small contact resistance, especially an electrode structure which enables an ohmic contact, and a method for manufacturing the same.
2. Description of the Prior Arts
So far various types of electrodes for a II-VI group compound semiconductor device have been studied. Haase et al., for example, have examined the applicability of Li, Na, Mg, Ti, Cr, Mn, Ni, Pd, Pt, Cu, Ag, Zn, Hg, Al, In, Sn, Pb, Sb or Bi and alloys thereof as electrode materials ("Short wavelength II-VI laser diodes", Inst. Phys. Conf. Ser. No.120 p.9). However electrode materials which provide ohmic contacts for II-VI group compound semiconductors have not yet been found.
Thus, Au is extensively used as an electrode metal, but since Au forms a Schottky junction with approximately 1.2 eV of potential barrier to p-type ZnSe, ohmic contacts have not yet been made.
In order to make the ohmic contact to, for example, p-type ZnSe, following methods are considered:
a low-energy-barrier intermediate layer of CdSe or HgSe is epitaxially grown between the electrode metal and p-type ZnSe, or
p-type ZnTe is used for the contact layer and a p-type ZnSeTe graded composition layer or an intermediate layer of a p-type ZnSe/ZnTe strained-layer superlattice is used between the p-type ZnSe and p-type ZnTe.
Otsuka et al. have demonstrated an ohmic contact of Au/p-CdSe and reported the possibility of that of Au/p-CdSe/p-ZnSe ("Growth and Characterization of p-type CdSe", Otsuka et al., Extended Abstracts (the 54th) p.255, The Japan Society of Applied Physics). Lansari et al. have made a good ohmic contact by growing HgSe on p-type ZnSe as a low-energy barrier intermediate layer by MBE and using Au as an electrode metal ("Improved ohmic contact for p-type ZnSe and related p-on-n diode", Y. Lansari et al., Appl. Phys. Lett. 61 p.2554). Fan et al. ("Graded bandgap ohmic contact to p-ZnSe", Y. Fan et al., Appl. Phys. Lett. 61 p.3160), and Hiei et al. ("Ohmic contact to p-type ZnSe using ZnTe/ZnSe multiquantum wells", F. Hiei et al., Electronics Lett. 29 P.878) have reported the fabrication of an ohmic contact by using p-type ZnTe for the contact layer and using a p-type ZnSeTe graded composition layer or the intermediate layer of a p-type ZnSe/ZnTe strained layer superlattice between the p-type ZnSe and p-type ZnTe.
However, none of the methods of making ohmic contacts to the conventional II-VI group compound semiconductors are satisfactory. They have the problems below.
When CdSe is used, a low CdSe acceptor concentration of 1.times.10.sup.17 cm.sup.-3 in CdSe makes it difficult to lower contact resistance. When HgSe is used, the sharing of the MBE apparatus used for forming other layers in the device manufacturing process brings deteriorated properties of devices because of the mixing of Hg atoms into other layers. Introducing exclusive MBE apparatus to avoid intermixing of atoms will conduct lower productivity. Furthermore, HgSe has poor chemical and physical stability.
When ZnTe is used, the stress remaining in the film because of large lattice mismatch between ZnSe and ZnTe may deteriorate the properties of devices, and it is difficult to keep ZnTe carrier concentration suitable. A large lattice mismatch between ZnSe and any of the above intermediate layers also causes strain, and the epitaxial growth lowers the productivity.
Furthermore, the Au electrodes used for the above methods are inferior in mechanical strength such as adhesion.
Accordingly, research was continued to create an electrode structure which makes an ohmic contact to II-VI group compound semiconductors, especially to p-type Zn.sub.x Mg.sub.1-x S.sub.y Se.sub.1-y (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) semiconductors.
FIG. 9 shows an ionized impurity concentration dependence of contact resistance of a metal/p-ZnSe Schottky junction as a parameter of the potential barrier .phi..sub.B between the metal and p-type ZnSe. As shown in FIG. 10, the potential barrier .phi..sub.B is given by .phi..sub.B =x.sub.s +E.sub.g -.phi..sub.M, in which x.sub.s represents an electron affinity of semiconductor, E.sub.g represents a bandgap of semiconductor and .phi..sub.M represents a work function of metal. FIG. 8 is a band diagram illustrating the Schottky barrier at a contact interface when a p.sup.+ -ZnSe layer lies between the metal and p-type ZnSe. FIG. 9 shows what is obtained by a calculation using the Yu model in which thermionic emission and tunneling currents are considered ("Electron Tunneling and Contact Resistance of Metal-Si Contact Barrier", A. Y. C. Yu, Solid State Electronics vol.13 p.239 (1970)). As a result, the contact resistance decreases with the increase of the ionized impurity concentration. This is due to the decrease of Schottky barrier width (w), as shown in FIG. 8, with increasing ionized impurity concentration, which results in the rapid increase of tunneling current.
This is also the same between metal and p-type Zn.sub.x Mg.sub.1-x S.sub.y Se.sub.1-y (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) semiconductors. For example, a figure showing the dependence of contact resistance on impurity concentration (i.e., corresponding to FIG. 9) shows a similar inclination in which the contact resistance differs in one figure against the same potential barrier parameter.
In other words, an ohmic contact can be made by using a intermediate layer having a high ionized impurity concentration on the p-type Zn.sub.x Mg.sub.1-x S.sub.y Se.sub.1-y (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) semiconductor layer surface, on which metal electrodes are formed.
However, p-type Zn.sub.x Mg.sub.1-x S.sub.y Se.sub.1-y (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1) semiconductor film is formed only by MBE, and the ionized impurity concentration is, at best in the order of 10.sup.17 cm.sup.-3. It is impossible to form a film with sufficiently high ionized impurity concentration to make an ohmic contact.